How Moisture Affects Hardwood Flooring

Hardwood flooring is a natural product that will respond to different humidity variations. This means that even though the wood has been dried, cut, and processed into flooring material, the hardwood may expand or contract because of moisture. A change in seasons is when these changes become especially noticeable. During warm and humid summers, hardwood flooring expands. During dry winter weather, hardwood flooring contracts. Because this seasonal movement is normal for hardwood products, the best way to reduce such movement is by installing humidity controls and making sure that they function before flooring is installed.

You may be thinking that it’s going to be impossible to avoid these types of distortions, but that’s not the case. In order to prevent your floor from expanding or contracting, it’s extremely important to keep up with hardwood flooring maintenance. With this in mind, you have to understand how these changes are caused based on the specific climate you live in as well as the climate of your home. But don’t worry, preventing movement of your hardwood flooring isn’t solely based on maintenance. There are also preventative measures that can be taken when your hardwood flooring is installed.

Wood is a hygroscopic material. Not sure what that means? That’s okay! When wood is exposed to air, it will either dry or pick up moisture until its reactions occur at equal rates with the humidity and temperature of the air. When the moisture is absorbed, the wood will swell. Wood will begin swelling as the moisture levels increase anywhere from 0% to 25-30%. When the moisture is lost, the wood will shrink. Shrinkage of wood begins at 25-30% of moisture content. Anything above 25%-30% means that your hardwood flooring is dimensionally stable.

Hardwood does not shrink or swell equally in all directions. A change in moisture content of a piece of hardwood flooring from 0% to 28% will increase the size of the piece approximately 0.1% along the LENGTH of the board, and 5% to 15% change in size in WIDTH of the strip with plain sawn flooring.

Winter Time

Love a nice fire and cozying up in blankets during winter time? Your hardwood flooring doesn’t. Because your home is normally heated during cooler months, your hardwood flooring gives up some of its moisture and contracts. Naturally, when this happens, thin gaps can appear between planks. As a homeowner, you should be prepared for this to occur. However, in order to try to prevent these separations, you can install a humidifier in the furnace or bring a movable humidifier into the room. As long as your humidity doesn’t fall lower than 40%, no gaps should appear between your hardwood flooring planks. Generally, once the weather starts to warm up for springtime, heating is turned down and humidity levels will naturally rise. In turn, most of the gaps in your hardwood flooring material will close up on their own.

Summer Time

When spring shifts to summer, your indoor humidity can rise up to 90%, causing your hardwood flooring product to expand. Even just a few days of humidity can cause wood flooring to cup, which means that the edges on the wooden board are higher than its center. Cupping can also happen when spilled water gets absorbed by your hardwood floors. To avoid cupping, keep your indoor humidity levels between 40-60%. High levels of humidity can cause hardwood flooring to expand significantly, sometimes causing the affected boards to lose their structural integrity and crack. That’s why it’s very important to never allow indoor humidity to rise over 65%. Your hardwood flooring maintenance can be made easy by keeping air conditioners or dehumidifiers running during the humid summer weather.


Wood Flooring Expansion and Contraction

Unlike many floor coverings, hardwood flooring material can last the lifetime of the building in which it is installed. As a homeowner, you should note that the number one enemy of your hardwood floor is moisture. Hardwood flooring naturally expands when moisture is present and shrinks when moisture is absent. Although not every reaction is a problem, stated below are some of the common results when water and hardwood flooring combine.

CRACKS BETWEEN BOARDS: When homes are heated, humidity levels plummet. In dry months, cracks can easily develop to the thickness of a dime on a typical solid 2¼ inch hardwood oak floor. These spaces in your hardwood flooring are to be expected and will usually close up as the seasons’ change and moisture returns to the air. If you want to try to combat these cracks between boards, try installing a humidifier in the furnace.

Cupping Hardwood Floor




CUPPING AND CROWNING: As with cracks between your hardwood flooring material, both cupping and crowning are also natural reactions to moisture. “Cupping” happens when the edges of a hardwood flooring board are higher than its center. Humidity is usually the culprit, although cupping can also happen if water is spilled onto the floor and absorbed into the hardwood flooring material. This moisture causes the wood to swell, crushing the boards together and deforming them at the edges. In order to repair your damaged hardwood flooring product, the cause of the moisture needs to be controlled. To speed up the drying process, use a fan and recoat your hardwood flooring with a flooring dealer approved finish once completely dry. This will leave your hardwood flooring material looking as good as new!

“Crowning”, on the other hand, is the opposite of cupping. Instead of the middle of the board being the highest point, the edges now surpass the center. This occurs when the surface of your hardwood flooring material encounters moisture. More often than not, this occurs when flooring has been sanded too soon after it has cupped.




BUCKLING: Be aware! Buckling is one of the most extreme reactions that can occur with your hardwood flooring. Fortunately, buckling is an uncommon occurrence, usually happening only after flood. Buckling happens when the floor pulls away from the subfloor, up to heights as high as several inches. Don’t worry, even in such cases it is still possible for hardwood flooring to be repaired rather than replaced.

Preventing Moisture Problems

Controlling humidity is the most important factor in preventing problems with moisture and your hardwood flooring material. Proper hardwood flooring maintenance will go a long way! To prevent moisture issues, it is best to:

  • Buy a “floor care kit” recommended by your hardwood flooring professional and clean your wood flooring with a cloth lightly dampened by the recommended cleaning product. Your flooring manufacturer will provide directions for use.
  • Clean your hardwood flooring material only when necessary. Do not clean your wood floors with water or water-based products on a regular schedule.
  • NEVER damp mop on a wood floor! The water deteriorates the wood and finish.
  • NEVER let a water spill dry on the floor.

Moisture and Exotic Wood Species

Hardwood flooring manufactured from exotic wood species is more demanding than when it’s made from domestic woods. Because of this, the indoor air humidity level must be in the range of 50-70% and air temperature should be kept in the range of 65°F – 75°F. To maintain your hardwood flooring at these requirements, use an air humidifier during the dry season and an air conditioner during humid summers. We recommend a combined digital temperature/moisture meter in order to monitor indoor conditions all year round.

To learn more about hardwood flooring products, available manufacturers, and installation techniques, please contact your local flooring expert today.


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